Musical instruments employing a vibrating mechanical element such as a string to produce sound have been provided heretofore with transducers commonly referred to as "pick-ups" for detecting the motion of the vibrating element and producing an electronic signal representing this vibration. This pickup signal may be amplified and converted to sound by a loudspeaker.
The sound produced by instruments of this nature dies out progressively after the string is excited. The sound can be prolonged if the pickup signal is amplified and coupled to the vibrating mechanical element of the instrument by acoustical, electromechanical, or electromagnetic feedback.
Acoustical feedback occurs when the energy in the sound output of the speakers strikes the vibratory element of the musical instrument, driving its vibrations. If the sound output is high enough the oscillations of the vibrating mechanical element will be sustained, However, this approach is effective only when the sound produced by the amplification of the loud speaker is extraordinarily loud.
Various attempts have been made to provide a "sustainer" or device capable of prolonging the notes independently of acoustical feedback from the environment in which an instrument is played. A typical, prior art sustaining device 11 is shown in FIG. 1A as including a magnetic pickup 10, a magnetic driver 12, and an amplifier 14 interposed in a circuit between the pickup 10 and the driver 12. The pickup 10 is typically comprised of one or more pickup coils, such as pickup coil 13. The driver 12 is typically comprised of one or more driver coils, such as driver coil 15.
The sustain system 11 may be used to sustain the vibrations of a single string, such as string 8, or a plurality of strings typically found on an electric guitar. The sustain system is usually disposed on a counter-sunk portion of the upper surface of the body of the electric guitar so that the pickup 10 and driver 12 are in magnetic proximity to the string 8 of the instrument.
The pickup 10 and driver 12 are constructed generally similarly. Both the pickup 10 and the driver 12 are constructed of a number of turns of a conductor means, such as wire 13, 15 which is wound around a magnetic core 17, 19, respectively. The cores 17, 19 are generally either a permanent magnet, or a ferrous material in contact with a permanent magnet, to provide a permanent magnetic flux through the center of the respective pickup coil 13 and driver coil 15.
A condition that exists in all prior art sustain systems using a magnetic pickup and driver in conjunction with an amplifier to sustain string vibrations is that when the gain of the amplifier 14 is of a sufficiently high level to achieve sustain of the string 8, a portion of the driver's 12 magnetic field F is present at the pickup 10. The magnetic field F induces the pickup 10 to create a voltage. The pickup voltage is amplified and regenerated by the driver 12, which then is picked up by the pickup 10, to induce the pickup 10 to create a greater voltage. This cycle is known as "system oscillation." The frequency of these oscillations depends on the resonant frequencies of the electronic components in the feedback loop and thus has no musical relationship to the string vibration frequency.
A second problem associated with direct magnetic feedback between the driver and pickup is the contamination of the pickup signal with noise and distortion produced by the amplifier means. The presence of amplifier noise and distortion in the pickup signal produces an unnatural tone when the pickup is used in conjunction with a loudspeaker to monitor the tone produced by the vibrating string.
One common solution to the problem of direct magnetic feedback is to decrease the gain of the amplifier. However, this decrease in amplifier gain reduces the ability of the system to pickup and sustain slight string vibrations. Also, it takes longer for the oscillations of the string to build up to their steady-state amplitude.
Another prior art solution to the problem of direct magnetic feedback is to spatially separate the pickup and driver by a great distance. The pickup and driver may be placed at opposite ends of the strings. However, this solution precludes the use of frets.
U.S. Pat. No. 4,075,921 presents a method for overcoming the problem of direct magnetic feedback by providing the pickup and driver with a very small air gap between the magnetic poles. The commercially available E-bow sustain system, manufactured by Gregory A. Heet of Los Angeles, Calif. embodies this type of approach. One difficulty with this approach is that the strings must be in very close proximity to the pickup and driver, and the string's vibrational excursion must be minimized to avoid direct contact between the strings and the pickup and driver.
U.S. Pat. No. 3,742,113. introduces a method for reducing the problem of direct magnetic feedback by providing the pickup with a hum-bucking apparatus to cancel the effects of uniform external magnetic fields. One difficulty with such an approach is that the hum-bucking pickup does not provide optimum rejection of the non-uniform magnetic field generated by the driver due to the balanced design of the pickup. As will be appreciated, the driver's magnetic field is non-uniform, particularly in close proximity to the driver due to the inverse cube law of magnetic field intensity. This law provides that the magnetic field intensity becomes more uniform as the distance from the driver is increased.
The '113 patent also discloses a sustaining device which has two drivers spaced equidistant from a single-coil pickup so that the field they exert at the locus of the pickup is substantially zero. However, the drivers are mounted in a shield box formed of magnetic ingot iron, which indicates that the equidistant positioning of the drivers with respect to the pickup, by itself, does not provide adequate magnetic field cancellation at the pickup. Although the magnitudes of the magnetic fields generated by the drivers may be equal at the pickup, the relative phases of the signals generated by the drivers will tend to vary with frequency from 180 degrees out of phase, thus the signals that the drivers induce in the pickup coil will not exactly cancel. This is due to the tendency that the driver coils will have different complex impedances from each other.
U.S. Pat. No. 4,941,388 discloses a method for overcoming the problems of direct magnetic feedback by unbalancing a hum-bucking pickup to provide optimum field cancellation of the non-uniform magnetic field generated by the driver. The magnetic field generated by the driver induces signals in both of the pickups of a hum-bucking pair. However, the magnetic field strength occurring at one pickup is stronger than the magnetic field strength occurring at the other pickup. This is because the pickups are not placed equidistant to the driver. Rather, one pickup is closer to the driver than the other pickup. Thus the signal amplitudes of the two signals must be equal in order for them to cancel when they are added together. This is achieved by altering the inductance of one of the pickups or by providing the appropriate gain or attenuation means to either or both pickup signals so that the amplitudes of the two signals are substantially equal. One difficulty with this approach is that the means of providing gain or attenuation to the signals changes their relative phase as well as their amplitudes. Although the amplitudes may be made substantially equal, the relative phase between the two signals will differ from 180 degrees and thus the signals will not adequately cancel, especially for higher signal frequencies.
A second difficulty with the unbalancing method proposed in the '388 patent is that in order to sustain a plurality of strings using multiple drivers and multiple pickups, wherein each string has an associated driver and unbalanced hum-bucking pickup, the geometry accorded by any positioning of these components guarantees that some magnetic field interaction will occur between the driver of each string and each of the pickups of adjacent strings. This magnetic interference precludes positioning the pickups in close proximity to the drivers, thus excludes a compact design for such a sustaining device.
The '388 patent also discloses a method for avoiding the problems of magnetic interference between adjacent sustaining systems by using either a single driver in conjunction with multiple pickups or a single pickup in conjunction with multiple drivers. However the use of a single driver or a single pickup for a multiplicity of strings inhibits the ability of the device to sustain more than one string simultaneously. If a single driver is used, the magnetic field generated to drive one string damps the motion of the other strings. A similar effect results from driving separate drivers with a single pickup signal. Furthermore, there is non-uniform gain in the feedback of the sustaining device between different fret-positions, different frequencies, and different strings. This sustaining device will sustain only the string whose signal feedback has the greatest gain.
Another prior art method for overcoming direct magnetic feedback is to make the sustain system operate more efficiently so as to reduce the power, hence the magnetic field strength generated by the driver needed to sustain string vibrations. Such a device is described by Floyd Rose et. al. in U.S. Pat. No. 4,907,483, U.S. Pat. No. 5,123,324, and U.S. Pat. No. 5,233,123. The sustainer that Rose describes is arranged to compensate for the phase lag of the drive forces provided by the driver relative to the drive signal. The efficiency of this system is most noticeable at higher frequencies. Although the increased efficiency is noticeable, the reduction in the power needed to drive string oscillations is not so significant in reducing the power requirements of the system as to substantially reduce magnetic feedback in nearby pickups.
A prior art sustaining device first marketed by the applicant and T Tauri Research of Wilmette, Ill. in November 1988 is shown in FIG. 1B as including a magnetic pickup 20, a magnetic driver 22, and an amplifier 24 interposed in a circuit between the pickup 20 and the driver 22. The pickup 20 includes a pickup coil 23 wrapped around a magnetic core 25. The driver 22 includes a driver coil 21 wrapped around a magnetic core 28. The driver 22 is positioned such that its axis is parallel to the string 8 and perpendicular to the axis of the pickup 20. The magnetic core 28 includes two pole pieces 27 and 29 that are in close proximity to the string 8.
Because the driver 22 is oriented perpendicular to the pickup 20, the amount of magnetic flux generated by the driver 22 at the pickup 20 is minimized. Furthermore, the magnetic flux acting on the pickup 20 is symmetric with respect to the plane that bisects the height of the pickup coil 23, thus the signals generated in the pickup coil 23 by the driver's 22 magnetic flux tend to cancel. Because the pole pieces 27 and 29 are in close proximity to the string 8, both ends of the driver solenoid are used to drive the string's 8 vibrations. Thus less power is needed to drive the string 8, resulting in a reduction in magnetic flux generated by the driver 22 that will interfere with the pickup 20. Although T Tauri's sustaining device is effective in reducing magnetic interaction between the driver 22 and the pickup 20, a significant amount of magnetic interaction still occurs. This magnetic interaction can be further reduced when the single-coil pickup 20 is replaced by a standard dual-coil hum-bucking pickup (not shown). However, the amount of magnetic interaction between the dual-coil hum-bucking pickup and the driver 22 still interferes with the operation of the sustaining device, especially where the hum-bucking pickup and driver 22 are placed close to each other.
Although the above described attempts to solve the problem of direct magnetic feedback all perform their intended function to one extent or another, room for improvement still exists.
Thus it is one aspect of the present invention to provide a sustaining device which maximizes the ability to sustain the vibration of a string, while minimizing the effects of direct magnetic feedback associated therewith. This is accomplished by two methods: One method is to provide an efficient sustain system capable of providing a powerful sustaining effect with only a modest power input to the driver. The second method is to balance the pickup means whereby the signals induced in the pickup means by the drive means will cancel.
It is another aspect of the present invention to provide a sustaining device whose pickup means and drive means are positioned in close proximity to each other without suffering the adverse effects of magnetic feedback.
It is yet another aspect of the present invention to provide a sustaining device that is capable of sustaining multiple strings simultaneously.